Muscovy ducks are different. Because ducks don't care what they mate with, you can only have 1 drake per 4-5 hens for all species- Muscovy or mallard-derived. Also, if a Muscovy mates with a Mallard-derived duck breed, they produce a sterile (can't reproduce, ever) breed called a "mule" duck- which are good only for meat, as they don't lay much, if any, eggs.